The Wave Reduction Efficiency How Relates with the Crest Widths, Working States of Pile-rock Breakwater in Coastal Mekong Delta

The coastline in Ca Mau and Kien Giang provinces on Mekong Delta has been severely eroded in recent years. The Pile-Rock Breakwater (PRBW) was selected as predominant structure to construct widely more than 30 km on the west coast of Ca Mau. This structure shows the effectiveness of wave reduction, stimulating sediment accumulation and facilitating restoration of mangrove forest. However, this breakwater had been designed on the basis of best-engineering practice, lacking sufficient scientific background with regard to the structural design, capacity of wave reduction, working conditions. This study is to investigate the interaction of crest width, working states (submerged, transition and emerged) and the wave transmission coefficient, wave reflection coefficient and wave energy dissipation of PRBW by experiment in the laboratory and derive the empirical formulas for this construction under different sea states and crest widths. The results show a significant influence of width dimensions on the above coefficients. The findings revealed that the crest width of the breakwater is inversely proportional to the wave transmission coefficient (Kt) under emerged state. The crest width is also proportional to the wave reduction efficiency and wave energy dissipation in both working states (submerged and emerged states). The front wave disturbance coefficient is proportional to the reflected wave coefficient and the wave height in front of the structure can increase by 1.4 times in the emerged state. It is especially important to note when designing this structure to reduce the erosion in the structure toe due to the reflected waves. The empirical equations including linear and non-linear formulas have also developed and compared with previous studies for different breakwaters. This is necessary to be considered to calculate the structure and stability of breakwater. The results of this study serve as the basis for the design of pile-rock breakwater to reduce the waves under different natural conditions in the coastal area of the Mekong Delta.